This disclosure relates to processes for making high temperature, high performance thermoplastic films and film laminate materials, particularly, isotropic liquid crystalline films and film laminate materials.
Liquid crystalline polymers are a family of materials that exhibit a highly ordered structure in the melt, solution, and solid states. They can be broadly classified into two types: lyotropic, having liquid crystalline properties in the solution state, and thermotropic, having liquid crystalline properties in the melted state. Most liquid crystalline polymers exhibit excellent physical properties such as high strength, good heat resistance, low coefficient of thermal expansion, good electrical insulation characteristics, low moisture absorption, and are good barriers to gas flow. Such properties make them useful, in sheet form, as substrate materials for printed circuit boards, packaging, integrated circuit (IC) chip packages circuitry and other high density applications.
Many of the physical properties of liquid crystalline polymers are very sensitive to the amount and direction of orientation of the liquid crystal regions in the polymer. The structure of the liquid crystalline polymer is easily ordered and oriented by shear forces occurring during extrusion, often leading to highly aligned liquid crystal chains that are retained in the solid state and result in highly anisotropic properties. Anisotropic properties are not desirable, however, in products having planar forms, such as tapes, films, sheets, and the like. Thus it is desirable, especially in circuit boards and other high density applications, to use a substantially or fully isotropic (non-ordered) liquid crystalline polymers.
A number of methods are used to produce liquid crystalline polymers in planar forms that have more balanced, less anisotropic properties. These include the use of multilayer flat dies that are oriented so as to extrude overlapping layers at intersecting angles, use of static mixer-agitators at the die inlets, and the like. More recently, dies having rotating or counter-rotating surfaces have been used. These extrusion techniques, used separately or in combination with other methods known in the art, such as film blowing, can produce liquid crystalline polymer film and sheet that are multiaxially oriented, that is, oriented in more than one direction, and have more balanced physical properties.
A characteristic of these methods is that locally, for example at the surfaces of the sheet or film, the molecules are oriented in the planar x-y directions by shear imparted at the extrusion surfaces. Thus, when examining the film in the z-direction, i.e., the thickness, the x-y orientation of the molecules will change progressively from one orientation (e.g., in the x direction) at one surface to another orientation (e.g., the y direction) at the opposite surface of the planar form. A drawback to the above described methods is that when attempting to make very thin multiaxially oriented films, e.g., films having a thickness of 25 micrometers or less, the forces imparted in the orientation transition region of the liquid crystalline polymer by the extrusion surfaces are exerted in increasingly opposing directions as the distance between the extrusion surfaces diminishes. This results in the formation of pinholes, tears, and other imperfections, for example, separation of surface layers (peeling) of the film. Additionally, such films are not fully isotropic.
Accordingly, there remains a need in the art for a process to produce liquid crystalline polymer films that are isotropic in the x-y plane, especially films with thicknesses less than about 25 micrometers.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a process to produce a liquid crystalline polymer film comprising depositing a fine powder of liquid crystalline polymer resin onto a carrier and fusing the deposited fine powder to form a liquid crystalline polymer film that is isotropic in the x-y plane. The liquid crystalline polymer resin powder may be applied by electrostatic deposition. The carrier can comprise an all-metal foil, a metal foil laminate, a polymer film material, or a release material.
In one embodiment, the carrier is removed to result in a free standing liquid crystalline polymer film. The free standing liquid crystalline polymer film may be subsequently applied to a substrate and laminated by heat and pressure.
In another embodiment, the carrier is not removed and the liquid crystalline polymer film is laminated to the carrier by heat and pressure.
In another embodiment the liquid crystalline polymer film/carrier material is applied to a substrate and laminated by heat and pressure to form a liquid crystalline polymer film disposed between the carrier and the substrate, often known in the industry as bi-cladded laminates.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.